1. Field of the Invention
The present invention relates generally to separating and measuring particles and molecules in fluids, and in particular, to using a radial cross-flow differential migration classifier to separate and/or measure particles.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by reference numbers enclosed in brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
For four decades, the long column differential mobility analyzer (DMA) has been the instrument of choice for the classification of aerosol particles in the 10 nm to 1 micron range [1]. However, there are currently no particle classifiers that are optimal in the sub-10 nm range. One reason is the residence time required for a classifier to effectively separate the particles.
The reduction of column length and the increase of flow rates have led to decreased residence times of samples in a classifier. This has enabled custom instruments to classify particles and gas ions in the range of 1 to 10 nm, where diffusive losses had hampered the performance of standard long column instruments [2-4]. Reductions in residence time have also been obtained by using a radial geometry, which has an inwardly accelerating flow [5-9].
However, in addition to residence time, another important consideration for mobility classifying instruments is its transport efficiency, which is the actual number of particles transmitted divided by the number of particles transmitted if there were no diffusive losses. A major cause for losses is the unfavorable field arising from the voltage transition that nearly all DMAs share [6]. The charged particles have an increased residence time in the neighborhood of the voltage transition. Parasitic fields resulting from static buildups on the dielectric can also further increase losses. Clever instruments have been developed that have eliminated the voltage transition altogether [10,11], but they have not been widely adopted; perhaps partially because of the complexity of determining their transfer function [10].
Altogether, current mobility classification instruments are generally bulky, require expensive blowers or pumps to achieve large flow rates, and require an unfavorable voltage transition at the inlet or outlet that hampers performance and transfer efficiency.
In view of the above, what is needed is a method, apparatus, and article of manufacture for continuously separating particles with enhanced transport efficiency and reduced loss of particles. In particular, there is a need for a method, apparatus, and article of manufacture for separating particles that reduces the diffusive loss of particles due to factors such as voltage transitions and static buildups.